Polymer extrusion systems are well known and used for applications such as the manufacture of extruded polymer components. In a typical application, polymer feedstock particles are combined and heated in an extruder device to produce a stream of molten polymer. The polymer extrudate is then driven under pressure to an appropriate downstream module. As one example, the polymer extrudate can be fed to a die to shape the polymer into a sheet, tube or other desired profile. In a typical polymer processing system, a filtration device is placed in-line between the extruder and the downstream module to filter the polymer extrudate and thereby improve its quality and uniformity. Typically, the filtration device includes two breaker plates that contain screens as filter elements. The filtration device typically includes an internal bore through which the polymer material flows, and a screen changing device that is movable across the bore to position one breaker plate in-line with the polymer flow while the other breaker plate is offline and thus accessible for cleaning or replacement.
The screen changing device thus necessarily interacts with the in-line bore of the filtration device. Moreover, the screen changing device is typically designed to perform a screen changing task during the operation of the processing system, and hence while polymer melt continues to flow through the filtration device. Accordingly, it is important that the filtration device be equipped with a suitable means for sealing the filtration device against the loss of polymer material from the bore. In particular, internal areas of the filtration device present the opportunity for polymer material to “hang-up” and stagnate, thereby reducing the quality and uniformity of the polymer material being processed.
Filtration devices in the past have offered various sealing approaches. Examples are disclosed in U.S. Pat. Nos. 6,270,703; 6,238,558; 6,168,411; 6,010,625; 5,783,223; 5,507,498; 4,470,607; 4,359,387; 4,237,014; 4,059,525; 3,962,092; and 3,675,934. U.S. Pat. No. 6,238,558 discloses a filter changer in which an upstream seal ring is adjustably compressed by screws oriented transversely to the direction of polymer flow. This design requires the use of a tool to adjust several screws, and the screws are of a nonstandard customized design that adds costs to the filtration device. U.S. Pat. No. 6,010,625 discloses an upstream seal that is constructed from a polymeric material. It is generally known that polymeric seals of this type are not suitable for use in environments where operating temperatures exceed 450–550° F. Moreover, the effectiveness of the seal requires polymer material flowing through the disclosed filtration device to be deliberately diverted into gaps, which is considered undesirable because the diverted polymer material can become degraded while stagnating in the gaps. U.S. Pat. No. 5,507,498 discloses a filtration device in which a C-shaped spring is used for sealing purposes. The C-shape of the spring thus requires a gap to exist between the cross-sectional ends of the spring. By design, some of the polymer material flowing through the filtration device migrates to the outer diameter of an upstream seal interacting with the spring, where such polymer material degrades and is then introduced back into the melt stream. U.S. Pat. Nos. 4,359,387 and 4,237,014 disclose a filtration device in which a Belleville-type spring is interposed between an upstream seal and an adapter component. It is believed that this design does not sufficiently prevent the leakage of polymer material. Other approaches, such as the O-rings, to enhance sealing within filtration devices do not sufficiently prevent polymer leakage and are not suitable for the high-temperature (e.g., 450–550° F. or greater) operating environments increasingly encountered in polymer processing applications.
In view of the foregoing brief discussion of prior art, it is widely accepted among persons skilled in the art that, despite past efforts, an ongoing need exists to provide filtration devices of the screen-changing type with improved sealing arrangements and methods.